Just pushed a new build from work @cpinder was looking at to improve performance.
In addition to trying with the latest build; if you're still having performance slowdowns, can you try these commands to see if they help? (reduces resolution of LoS/FoW/blurring in return for performance)
/imagequality [0-3]
(where 0 is default, and higher numbers are reducing resolution 75/50/25%)
Thanks,
JPG